Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (I/O modules), disk drives, and disk interface units (disk adapters). The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to any one of the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
In some cases, it is possible to provide redundant storage that maintains a mirror copy of data provided by a host computing system. In such systems, the redundant storage devices act as a single logical storage device for any computing devices coupled thereto. Writes are provided to one of the storage devices and are automatically copied to the other storage device for mirroring. Data can be read from either storage device. If one of the storage devices fails, the other storage device maintains operations automatically without having to manually switch or reconfigure the system. In some cases, the failed storage device can be replaced without the need to interrupt operations. However, some systems, such as systems provided by IBM Corporation, are not inherently capable of communicating with separate physical storage devices configured as a single logical device. These systems contain safeguards to prevent such a configuration, which mimics host to storage device coupling errors.
Accordingly, it is desirable to provide a mechanism that allows for redundant storage devices that can be maintained as a single logical device in systems that would otherwise not allow such a configuration.